app

FILE

app

FILE SUMMARY

Application resource file.

DESCRIPTION

The application resource file specifies the resources an
application uses, and how the application is started. There must
always be one application resource file called
Application.app for each application Application in
the system.

The file is read by the application controller when an
application is loaded/started. It is also used by the functions in
systools, for example when generating start scripts.

For the application controller, all keys are optional.
The respective default values are used for any omitted keys.

The functions in systools require more information. If
they are used, the following keys are mandatory:
description, vsn, modules, registered
and applications. The other keys are ignored by
systools.

Warning

The RTDeps type was introduced in OTP 17.0 and
might be subject to changes during the OTP 17 release.

description

A one-line description of the application.

id

Product identification, or similar.

vsn

The version of the application.

modules

All modules introduced by this application. systools
uses this list when generating start scripts and tar files. A
module can only be defined in one application.

maxP

Deprecated - will be ignored
The maximum number of processes allowed in the application.

maxT

The maximum time in milliseconds that the application is
allowed to run. After the specified time the application will
automatically terminate.

registered

All names of registered processes started in this
application. systools uses this list to detect name
clashes between different applications.

included_applications

All applications which are included by this application.
When this application is started, all included application
will automatically be loaded, but not started, by
the application controller. It is assumed that the topmost
supervisor of the included application is started by a
supervisor of this application.

applications

All applications which must be started before this
application is allowed to be started. systools uses
this list to generate correct start scripts. Defaults to
the empty list, but note that all applications have
dependencies to (at least) kernel and stdlib.

env

Configuration parameters used by the application. The value
of a configuration parameter is retrieved by calling
application:get_env/1,2. The values in the application
resource file can be overridden by values in a configuration
file (see config(4)) or by command line flags (see
erl(1)).

mod

Specifies the application callback module and a start
argument, see application(3).

The mod key is necessary for an application
implemented as a supervision tree, or the application
controller will not know how to start it. The mod key
can be omitted for applications without processes, typically
code libraries such as the application STDLIB.

start_phases

A list of start phases and corresponding start arguments for
the application. If this key is present, the application
master will - in addition to the usual call to
Module:start/2 - also call
Module:start_phase(Phase,Type,PhaseArgs) for each
start phase defined by the start_phases key, and only
after this extended start procedure will
application:start(Application) return.

Start phases may be used to synchronize startup of an
application and its included applications. In this case,
the mod key must be specified as:

{mod, {application_starter,[Module,StartArgs]}}

The application master will then call Module:start/2
for the primary application, followed by calls to
Module:start_phase/3 for each start phase (as defined
for the primary application) both for the primary application
and for each of its included application, for which the start
phase is defined.

This implies that for an included application, the set of
start phases must be a subset of the set of phases defined
for the primary application. Refer to OTP Design Principles for more information.

A list of application versions that the application
depends on. An example of such an application version is
"kernel-3.0". Application versions specified as runtime
dependencies are minimum requirements. That is, a larger
application version than the one specified in the
dependency satisfies the requirement. For information on
how to compare application versions see
the
documentation of versions in the system principles
guide. Note that that the application version
specifies a source code version. An additional indirect
requirement is that installed binary application of
the specified version has been built so that it is
compatible with the rest of the system.

Some dependencies might only be required in specific runtime
scenarios. In the case such optional dependencies exist, these are
specified and documented in the corresponding "App" documentation
of the specific application.

Warning

The runtime_dependencies key was introduced in
OTP 17.0. The type of its value might be subject to changes during
the OTP 17 release.

Warning

All runtime dependencies specified in OTP applications
during the OTP 17 release may not be completely correct. This
is actively being worked on. Declared runtime dependencies in OTP
applications are expected to be correct in OTP 18.